With rapid development of information industry, unprecedented challenges have been brought to future mobile communication technologies by increasing requirements of mobile Internet and internet of things (IoT). For example, based on a report of International Telecommunications Union (ITU), e.g., ITU-R M.[IMT.BEYOND 2020.TRAFFIC], it is expected that by 2020, growth of mobile services will be nearly 1000 times compared with mobile services in 2010 (4G era). Number of connected user equipment (UE) may exceed 17 billion. Number of connected devices may be even more amazing, when vast amounts of IoT devices gradually penetrate to mobile communication network. In response to the unprecedented challenges, broad research on the fifth mobile communication technologies (5G) have already been conducted by communication industry and academic community, to face with 2020. In the report ITU-R M.[IMT.VISION] of the ITU, framework and overall objectives of future 5G have been discussed. Such report also provides detailed descriptions about demand outlook, application scenarios and various important performance indexes of 5G. For new demands in the 5G, the report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] of the ITU provides relevant information about tendency of the 5G technologies, which aims to solve the following significant problems, such as support IoT, time delay, energy efficiency, cost, network flexibility, emerging service support and flexible spectrum utilization, by using significant improvement of system throughput, user experience consistency and extensibility.
Modulation waveform and multi-access mode are important bases of Air-interface of wireless communication, and 5G is no exception. At present, a typical example, Orthogonal Frequency Division Multiplexing (OFDM) of Multi-Carrier Modulation (MCM) technical family has been widely used in broadcast audio and video field, as well as civil communication systems, e.g., Long Term Evolution (LTE) system, which corresponds to Evolved Universal Terrestrial Radio Access (E-UTRA) established by 3rd Generation Partnership Project (3GPP), European Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB), Very-high-bit-rate Digital Subscriber Loop (VDSL), IEEE802.11a/g Wireless Local Area (WLAN), IEEE802.22 Wireless Regional Area Network (WRAN) and IEEE802.16 World Interoperability for Microwave Access (WiMAX), and so on. Basic idea of the OFDM technologies is to divide a broadband channel into multiple parallel narrow band sub-channels/sub-carriers, to enable high-speed data stream transmitted in a frequency selective channel to be converted into low-speed data stream, which is transmitted in multiple parallel independent and flat sub-channels. Thus, system capability for resisting multipath interference may be greatly improved. And the OFDM may utilize Inverse Fast Fourier Transform (IFFT)/Fast Fourier Transformation (FFT) to implement simplified modulation and demodulation modes. Secondly, by adding a Cyclic Prefix (CP), linear convolution with the channel may be converted into circular convolution. Subsequently, based on properties of the circular convolution, when a CP length is greater than a channel maximum multipath delay reception without Inter-symbol Interference (ISI) may be implemented by using a simple single-lap frequency domain equalization, so as to reduce processing complexity of receiver. Although Mobile Broadband (MBB) service requirements of 4G era may be well supported by CP-OFDM modulation waveform, there may be great limitations or deficiencies for the CP-OFDM in 5G scenarios as follows, due to the fact that 5G may be faced with more challenging and more rich scenarios.
(1) In a low delay transmission scene of 5G, when adding the CP to resist the ISI, spectrum utilization may be greatly reduced for the following reasons. Symbol length of the OFDM may be greatly shortened by low-delay transmission, while a CP length is only limited by impact response of channel. Subsequently, a ratio of CP length to OFDM symbol length may be greatly increased. Such overhead may lead to huge spectral efficiency loss, which is difficult to be accepted.
(2) In the IoT scene of 5G, a larger signaling overhead needed by closed-loop synchronous maintenance may be resulted from strict time synchronization requirements. Besides, there may be no flexibility in data frame structure, when a strict synchronous mechanism is employed. Subsequently, different synchronization requirements of various services may not be well supported.
(3) The OFDM employs Rectangular Pulse, which enables frequency domain side lobe of the OFDM to roll down very slowly, and causes a great deal of out-of-band leakage. Thus, the OFDM is very sensitive to Carrier Frequency Offset (CFO). However, 5G may have a lot of requirements for fragmentation spectrum to flexibly access, or to be shared. Flexibility of spectrum access may be greatly limited by high out-of-band leakage of the OFDM. Alternatively, in other words, a large frequency domain protection band is needed. Thus, spectrum utilization may be reduced.
The foregoing deficiencies are resulted from intrinsic characteristics of the OFDM. Although impacts resulted from these deficiencies may be reduced by taking some measures, meanwhile, the complexity of system design may be increased, which could not solve problems fundamentally.
For the foregoing reasons, as described in report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] of the ITU, some new waveform modulation technologies (based on multicarrier modulation) have been considered by the 5G. Filter Bank Multicarrier (FBMC) modulation technologies become one of hot research subjects. Since degree of freedom of Prototype Filter design has been provided, a filter with better Time/Frequency Localization (TFL) characteristics may be employed to perform pulse shaping to transmission waveform, to enable transmission signals to display various better characteristics, e.g., CP is not needed to resist the ISI and spectral efficiency may be improved, flexible fragmentation spectrum access may be well supported by lower out-of-band leakage, and characteristics not sensitive to CFO. To achieve maximum spectral efficiency, a typical FBMC system may generally employ Offset Quadrature Amplitude Modulation (OQAM) technologies. Thus, the foregoing technologies are generally referred to as FBMC/OQAM system, which may also be referred to as OFDM/OQAM system. An early literature “Analysis and design of QFDM/QQAM systems based on filter bank theory”, IEEE Transactions on Signal Processing, Vol. 50, No. 5, 2002 may be simply referred to, when wondering how the FBMC may be applied to digital communication.
The FBMC has gotten attention in 5G research, due to better characteristics thereof not possessed by the OFDM. There are many challenges to be faced with, when the FBMC is used in applications of wireless communication system, due to some intrinsic deficiencies of the FBMC. These challenges needing to be urgently solved are continuously studied. One problem therein is intrinsic interference problem of the FBMC/OQAM system. To use prototype filter with better TFL characteristics. Inter-carrier Interference (ICI) may generally exist between adjacent sub-carriers of the FBMC system. Meanwhile, ISI may be resulted from long tail in time-domain. Intrinsic interference problems of the FBMC system may be resulted from simultaneous presence of ICI and ISI. The FBMC/OQAM has introduced adjacent carriers and real-number-field orthogonality between symbols, by respectively modulating real number symbols on real/imaginary part of adjacent subcarriers, and transmitting alternately in time. Subsequently, ICI and ISI may be eliminated perfectly in a scene without fading. However, the real-number-field orthogonality may be destroyed by fading channel. Subsequently, information transmitted on subcarrier and symbol may be leaked to adjacent subcarriers and symbols. The foregoing interference may not be reduced, accompanying with increasing Signal-Noise Ratio (SNR), which may lead to degraded system error performance. In a scene with higher SNR, system is in an interference limited environment, and error performance may be mainly determined by interference. Thus, the ICI and ISI may lead to an error floor with higher SNR. In a high-speed mobility environment (such as a high-speed train with a strong time selective environment), or a strong frequency selective environment, higher error floor resulted from intrinsic interference of the FBMC/OQAM system may be even worse, which may also have great impact on system link reliability. Since interference is relevant to all the adjacent subcarriers, elimination operations may be relatively complicated, when only eliminating interference at the receiver. Although the foregoing problem may be effectively dealt with by using a method, such as Successive Interference Cancellation (SIC), implementation complexity thereof may be relatively higher, which may also lead to longer delay. Besides, there are higher requirements for the receiver to learn accurate channel state information.
In view of above, to improve competitiveness of the FBMC in the 5G candidate technologies, in addition to developing advantageous features of the FBMC, deficiencies thereof are also needed to be solved. Regarding various scenarios in 5G, particularly data communication in high-speed mobility environment and stronger frequency selective environment, it is necessary to solve problem of high error floor resulted from intrinsic interference of the FBMC system by using an effective method, so as to improve reliability of system links.